Wandering Lonely as a Planet

The classic theory of how planets are born takes a big hit from a paper in
today's issue of Science. There
researchers from Spain, the U.S. and Germany report the discovery of 18 planet-size
objects that, instead of circling a single star, drift free in the Orion
constellation. Most planets are thought to form over tens of millions of
years, as the gas and dust swirling around one star slowly condenses into distinct
clumps. But the new objects have no central star, and their nearest cluster
is a mere five million years old. "The formation of young, free-floating, planetary-mass
objects like these is difficult to explain," said lead author Maria Rosa Zapatero
Osorio of the Instituto de Astofisica de Canarias.

Most extrasolar planet hunters
would have missed these new candidates based on the way they search: to find
a planet, they look for evidence of its gravitational tug on the star it's orbiting--namely
a little starlight wobble. But Zapatero Osorio and her colleagues instead detected
dim, red light emitted directly from the objects in question, using visible-
and infrared-light telescope cameras in Spain, the Canary Islands and Hawaii.
Thus, they were also able to collect spectrographic information indicating the
objects' compositions. And because heavier molecules form under cooler conditions,
they learned of the objects' temperatures as well.

"The spectrographic results corresponded to our expectations that these were
young giant planets," Zapatero Osorio said. The researchers further estimated
the objects' masses to be between five times the mass of Jupiter and 15, depending
on their age. Generally, bodies less than 13 Jupiter-masses are considered planets,
whereas those between 13 and 75 Jupiter-masses are called brown
dwarfs. Although the new candidates could be either, Zapatero Osario notes
that earlier brown dwarf surveys suggest that finding 18 of them in one small
area is highly unlikely. "This is only a problem of terminology," she said.
"If planets can only exist around a star, then our candidates are very low-mass
brown dwarfs. But if planets must be a certain mass, then these objects are
planets."